It’s set in 1983. “Common LISP The Language” was published in 1984.
(The ANSI standard was 1994. Still nowhere near 20 years).
Well, 10 years is the same order of magnitude as 20.
Not really. If you were at MIT in 1983 then one would have probably access to Maclisp, whose compiler had seen some work by then. The first Scheme implementation was also written in Maclisp.